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The fuse called a fast-acting fuse is ignited
with a flame or a spark -
The gunpowder explodes, creating lots of heat and gas that
cause a buildup of pressure beneath the shell. -
Then, when the pressure is great enough, the
shell shoots up into the sky. -
After a few seconds, when the aerial shell is high above the ground, another fuse inside the aerial shell, called a time-delay fuse, ignites, causing the bursting charge to explode.
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This, in turn, ignites the black powder and the stars, which rapidly produce lots of gas and heat, causing the shell to burst open, propelling the stars in every direction.